The present invention relates to a tensioning system and, more particularly, a tensioning system for supporting a marine element such as a riser extending between a subsea base and a surface termination.
Tensioning systems are required to maintain a substantially constant tension in such vertical members despite the effects of wave and current on the floating superstructure which continually shifts, shortening and then lengthening the distance between the base fixed on the sea floor and the moving superstructure. The need for a constant tension, motion compensating device varies with the application. Thus, the compensation may serve to limit the load on vertical mooring lines such as cable tethers of a TLP or avoid excessive tension, compression or bending loads on tubular goods such as risers.
Risers connecting surface facilities with a subsea base present a particular problem in offshore drilling and production systems including drill ships, semi-submersible vessels and other non-bottom founded designs. Even some bottom founded platform designs such as articulated or compliant towers may have sufficient movement between topside facilities and the riser to require compensation. Uncompensated support may allow the riser to build a net compressive load sufficient to buckle the riser, collapsing the pathway within the riser necessary for drilling or production operations. Alternatively, excess tension from uncompensated support can also damage the riser.
Further, the relative motion between risers and surface facilities is a problem even for oil and gas operations from tension leg platforms (xe2x80x9cTLP""sxe2x80x9d) which are designed to minimize the wave response of the floating superstructure.
An object of the present invention is to provide a tensioning system for supporting one end of a marine element from a superstructure which is in relative motion therewith. It is a further object of the present invention to provide favorable dynamic responses of applied force and stroke length to such a tensioning system in its support of a marine element.
Another object of the present invention is to provide a tensioning system for maintaining oil and gas production risers in substantially constant tension in offshore applications in which one end of the riser is fixed at the sea floor and the other end of the riser is secured to a moving superstructure through the tensioning system.
It is a further object of the present invention to provide a riser tensioning system which facilitates ease of offshore maintenance and/or replacement of the tensioning controlling members.
Finally, it is an object of the present invention to provide a system to accommodate the use of a fixed derrick on an oil and gas platform from which a plurality of wells will be drilled by offsetting the platform, the device facilitating the acceptance of production risers in tensioning equipment offset from the drilling facilities.
Toward the fulfillment of these and other objects according to the tensioning system of the present invention, a tensioning system is provided for supporting marine elements such as risers which extend from a fixed lower end at a subsea base or foundation to a moving, floating superstructure. The tensioning system has a lever arm pivotally connected to both the superstructure and the upper end of the marine element and a tension controlling strut member pivotally connected to both the superstructure and the lever arm. This aspect of the tensioning system facilitates control of dynamic response by deploying tension controlling elements which are limited in force and movement to their optimal range, yet afford matching of applied force and stroke length for an offshore application through the lever arm configuration.
Another aspect of the present invention facilitates maintenance and replacement of tension controllers with the use of a mounting ramp to provide initial tension in a mechanical spring embodiment.
A further aspect of the present invention is an improved pressure charged elastomeric energy cell.